This invention relates to a procedure as well as an apparatus for the cleaning of flue gases containing sulfur dioxide that come from circulating fluidized-bed firing systems.
Circulating fluidized-bed firing systems are used in particular for the low-emission combustion of fossil fuels, e.g. coal, peat, wood, and so forth. In burning sulfur-containing coal, for example, the oxidation of the sulfur produces sulfur dioxide, which gets into the atmosphere via the flue gas. These emissions, which are harmful to the earth""s atmosphere, are returned to the earth as acid rain by way of the weather cycle. Various procedures have been developed for reducing these harmful emissions to the greatest possible extent.
An overview of the retaining of sulfur dioxide in fluidized-bed firing systems was presented by E. J. Anthony during the xe2x80x9cMediterranean Combustion Symposiumxe2x80x9d in Antalya, Turkey in June 1999.
For example, a familiar procedure is the addition of a fine-grained alkaline SO2 sorbent, generally limestone (CaCO3), burnt lime (CaO) or also dolomite, into the combustion chamber of the fluidized-bed firing system. Here, first of all the roasting (calcining process) of the limestone to burnt lime (CaO) takes place, and subsequently a reaction occurs between the roasted limestone and the sulfur dioxide of the flue gas.
If in this connection the limestone is exposed to the temperatures of 700xc2x0 C. to 950xc2x0 C. that are present in a circulating fluidized-bed firing system, namely if carbon dioxide is driven off from the limestone, then what remains is burnt lime, which because of the driving off of the CO2 has a high degree of porosity and thus a high specific surface area.
The subsequent gas-solid reaction of the burnt lime (sorbent) with sulfur dioxide and oxygen is a surface reaction, and this is why the creation of a high specific surface area is a fundamental prerequisite for this reaction. Remaining behind as a solid reaction product is calcium sulfate or gypsum (CaSO4), which stays in the pores or on the surface of the sorbent or the burnt lime.
Depending on the grain size of the limestone or SO2 sorbent used and on its abrasion properties, either the aggregate of the sorbent-reaction product (lime-gypsum aggregate) remains long enough in the combustion chamber for it to be drawn off via the components of the combustion-chamber ash removal system, or in the case of small particles the sorbent-reaction product aggregate leaves the combustion chamber together with the flue-gas stream and is subsequently separated out in the following flue-gas filter.
The mixture composed of fuel ash, reaction product, and free, unreacted sorbent that is drawn off via the combustion-chamber ash removal system is generally referred to as bottom ash or coarse ash. The particle size of this coarse ash is for the most part larger than 100 xcexcm. The maximum grain diameter can amount to several mm.
The ash carried off with the flue gas that is subsequently separated out in the filter is generally called filter ash. Depending on the quality of the cyclone/separator, the grain size of this ash encompasses the small grain fractions up to about 200 xcexcm in diameter.
From knowledge gained by constructing fluidized-bed firing systems, it is evident that for the degree of desulfurization required in industrial use, namely a reduction in sulfur dioxide of from 70% to 99%, the desulfurization reaction requires a high excess of sorbent. This requirement is all the higher the greater is the demand placed on the degree of desulfurization.
If one uses the Ca/S ratio as a measure for the added limestone or another sorbent, namely the molar quotient of externally supplied Ca and the total sulfur of the fuel that is present, then typical Ca/S values for fluidized-bed firing systems lie between 2 and 4 for a degree of desulfurization of 95%.
This requirement has economic disadvantages, in particular because in general this raises not only the operating costs for the procurement of limestone or of another sorbent, but also the waste-disposal costs for the resulting ash due to the fraction of unreacted sorbent.
In connection with the above-named desulfurization procedure for the flue gas in a circulating fluidized-bed firing system, it has proved to be a shortcoming that the limestone or the SO2 sorbent does not react completely with the sulfur dioxide, since frequently a blanket of gypsum that is almost gas-impermeable forms around the lime aggregate or sorbent aggregate, and also the pores of the lime or sorbent are clogged up by gypsum as the reaction product. The physical/chemical basis for this is the larger molar volume of SO2 diffusing into the lime aggregate or sorbent aggregate compared to the expelled CO2.
Especially in the interior of the grains of the sorbent-reaction product (lime-gypsum grains), a core of unreacted sorbent remains that is no longer available for the reaction, since the reaction partners of sulfur dioxide and oxygen can no longer penetrate down into this core.
The concentrations of unreacted free sorbent in the ash mixture can be as much as 40%, both in the case of filter ash and also with coarse ash, relative to the total ash mixture that is to be carried off. Also, within the framework of the further use of the ash mixture in the cement industry or in roadbuilding, it is desirable to have a lower concentration of free, namely unreacted, sorbent or limestone, to below 3 to 5%.
In current fluidized-bed firing systems, various techniques are being used at present in order to increase the degree of utilization of sorbents or limestone for the purposes of reducing the sulfur dioxide.
Thus, for example, in circulating fluidized-bed firing systems the ash accumulating in the flue-gas filter, which may still contain high proportions of unreacted sorbent, is returned again directly into the combustion chamber.
One drawback of this ash recycling is that the utilization of the still free sorbent in the ash may be of only limited benefit, because the additional dwell time in the fluidized-bed combustion chamber is small and because the reactivity of this ash or of the free sorbent contained in the ash is considerably reduced compared to the original sorbent.
Moreover, a recycling of the bed ash drawn off from the combustion chamber is also customary. To this end, the bed ash is subjected in part to a treatment (sifting of the good grain fraction or grinding up of the bed ash) that is aimed at increasing its degree of reactivity. But this method as well has the drawback that its effect in reducing the requisite consumption of sorbent is very limited, since it does not eliminate the cause of the incomplete reaction, the above-mentioned gypsum blanket around the sorbent or lime aggregate.
By way of the document U.S. Pat. No. 4,312,280, Shearer et al., a system has furthermore been disclosed in which ash from stationary fluidized-bed firing systems is brought into contact with water or steam and is returned to the combustion chamber of the fluidized-bed system. The mixing of the ash with steam and water takes place in a complex fluidized-bed reactor at relatively high temperatures. No reference is made to more extensive process-technology details about operating temperatures of this fluidized-bed reactor or to what water admixtures are used for doing this work. This disclosed system has on the whole a high technological complexity and therefore has not gained much acceptance on the market, partly also because the potential market for stationary fluidized-bed firing systems is limited to small system sizes, and compared to circulating fluidized-bed firing systems they have the disadvantage of having smaller particle dwell times as well as an inhomogeneous temperature distribution.
The object of the invention, then, is to devise a procedure as well as an apparatus for the cleaning of flue gases containing sulfur dioxide that come from circulating fluidized-bed firing systems, in which the above-mentioned disadvantages are avoided. Furthermore the efficiency or the degree of utilization of the sorbent used is to be increased and thereby the quantity of sorbent needed for the sorption of the sulfur dioxide is to be reduced.
Through the achievement in accordance with the invention, a procedure as well as a mechanism are provided that have the following advantages.
By the mixing together of the ash mixture coming from the combustion chamber of the circulating fluidized-bed firing system (ash, reaction product, and unreacted SO2 sorbent) with water or with an aqueous, sodium-containing solution in a mechanical mixing unit at a reaction temperature of 60xc2x0 to 100xc2x0 C. and at atmospheric pressure, and by recycling this into the combustion chamber of the circulating fluidized-bed firing system, the degree of utilization of the sorbent is considerably increased compared to a simple recycling of filter ash or bottom ash.
This effect arises from the fact that due to the mixing together of the ash with water or with an aqueous, sodium-containing solution in a manner corresponding the procedure in accordance with the invention, the still unreacted sorbent is first caused to react at a reaction temperature of 60 to 100xc2x0 C. with water or with an aqueous, sodium-containing solution to form a hydration product. This reaction is exothermic. The elevation in temperature as well as the reaction rate depend on the concentration of the unreacted sorbent in the ash, the temperature of the supplied ash, and the temperature of the supplied water as well as on the parallel reaction with the SiO2 and Al2O3 contained in the ash mixture. Since the hydration product has a lower density than the sorbent, this reaction causes the sorbent grain to xe2x80x9cswell,xe2x80x9d so that the adsorbate blanket (or gypsum blanket) around the sorbent aggregate gets broken up.
In this connection it has turned out that the degree of conversion of the SO2 sorbent into hydration product as well as the reaction rate increase considerably with increasing temperature, and within the range of about 60xc2x0 C. to about 100xc2x0 C. and at atmospheric pressure they reach a value that is optimally desirable in such an operation. If the temperature is too low, then the formation of hydration product proceeds only very slowly and is not complete within the dwell time that is available in the mixing unit. If the temperature is too high, then a xe2x80x9cboilingxe2x80x9d of the ash mixture takes place, with the consequence that due to the excess reaction enthalpy, additional water is evaporated, which is then no longer available for the hydration reaction. The consequences of reaction temperatures that are too high are an increased water consumption and problems associated with the additional vapor formation.
The reaction temperature in the mixing unit is regulated in an expedient manner in order to achieve an optimal conversion of unreacted SO2 sorbent into a hydration product. This enables the dosed-out quantity of water or of sodium solution to be adjusted in accordance with the concentration of the unreacted SO2 sorbent in the ash. Thus via a thermodynamic balance the optimal quantity of water to be added can be determined.
In another advantageous embodiment of the invention, the desired reaction temperature in the mixing unit is achieved by adding preheated water into the mixing unit, with the water temperature being regulated by a preheater located in front of the mixing unit. By the preheating of the water, the reaction temperature in the mixing unit can be set within the temperature range that is favorable to the course of the reaction, independently of the concentration of the non-reacted sorbent in the ash. Instead of water, an aqueous, sodium-containing solution can also be supplied.
Through the regulation of the quantity of water or of aqueous sodium-containing solution that is fed to the mixing unit, the mixture product can be carried off from the mixing unit in an advantageous manner as a function of the residual moisture. In this way, the product that is to be carried off can be produced in a desirable fashion.
In one advantageous embodiment of the invention, the dwell time of the product introduced into the mixing unit is regulated as a function of the degree of hydration of the product to be carried off. It is especially advantageous when the minimum dwell time in the mixing unit and/or the subsequent delivery lines amounts to one minute, in order to ensure that the hydration of the introduced product occurs in the desired manner.
It is expedient for the product drawn off from the mixing unit to exist in the form of a solid and to have a residual moisture less than 10%. This keeps the product drawn off from the mixing unit from being sludgy and thus difficult to transport due to a too-high excess of water.
It may be advantageous to construct the mixing unit in two stages, where in the first stage a portion of the water or of aqueous, sodium-containing solution required for the mixing is admixed with the ash, the reaction product, and the unreacted sorbent, and in the second stage the remaining portion of water or aqueous, sodium-containing solution is admixed in a regulated way as a function of the residual moisture of the product to be carried off from the mixing unit. In this way, in the first mixer attention can be directed toward the mixing process and in the second mixer it can be directed toward the requisite dwell time as well as to the temperature requirements.
By feeding the product carried off from the mixing unit into a drier, this product can be stored in an advantageous way after it has been dried. Thereby, in another advantageous embodiment of the invention the possibility is provided of putting this product into intermediate storage and feeding it to the combustion chamber after a certain interval of time.
It is further advantageous to return at least a portion of the product carried off from the mixing unit back into the mixing unit. With this measure, the dwell time for the reaction within the mixing unit can likewise be affected.
By feeding the ash mixture into a sifting/sizing unit before introducing it into the mixing unit, grain sizes of undesirable magnitude, for example larger than 300 microns, can be sifted out. This makes it possible to largely avoid erosions within the mixing unit as well as in the delivery lines. The same effect can be achieved by directing relatively large and undesirable grain sizes into a grinding unit and subsequently passing these on to the mixing unit.
In one advantageous embodiment of the invention, the ash, the reaction product, and the unreacted SO2 sorbent drawn off from the circulating fluidized-bed firing system and fed to the mixing unit is supplied to the mixing unit from the flue-gas filter as filter ash and from the combustion chamber as bottom ash over separate supply lines in each case, in which connection either filter ash or bottom ash or an adjustable mixture of the two can be fed to the mixing unit. This makes it possible to respond to any operational situation of the fluidized-bed firing system.
Furthermore the apportionment of the mixture of filter ash and bottom ash can be set by adjusting the grain sizes of added fuel and SO2 sorbent.
It is advantageous to introduce the water or an aqueous, sodium-containing solution into the mixing unit by means of at least one nozzle. As the situation demands, the requisite nozzles can be installed within the mixing unit at any points desired.
It is advantageous to use limestone as the SO2 sorbent. This is relatively inexpensive and has worked well as an SO2 sorbent. Furthermore it may also be advantageous to use dolomite as the SO2 sorbent.
In another advantageous embodiment of the invention, 50% to 500% of solid mixture, relative to the solid mixture normally leaving the combustion chamber of the fluidized-bed firing system, is fed to the mixing unit for the hydration process and subsequently fed again to the combustion chamber. Thereby an optimal conservation of the needed SO2 sorbent is achieved.
In one special case of the procedure in accordance with the invention, a portion of the SO2 sorbent is delivered directly to the mixing unit. In this way, regardless of the amount of ash in circulation as well as unreacted SO2 sorbent, the requisite amount of SO2 sorbent can be influenced from the outside.
In another special case of the procedure in accordance with the invention, a hydration product or Ca(OH)2 can be delivered into the line between the mixing unit and combustion chamber, with it being advantageous for this product to be delivered upstream of an intermediate storage, as seen in the direction of flow of the product. In this way, this quantity of product can likewise be influenced from the outside.
If an aqueous, sodium-containing solution is used, it is advantageous for the solution to contain sodium in the form of ions on an order of magnitude of up to 3% by weight, relative to the unreacted SO2 sorbent.
Furthermore it may be advantageous for the mixing unit to include an additional feed-in of water or of an aqueous, sodium-containing solution, by means of which water or an aqueous, sodium-containing solution is added to the ash, the reaction product, and the unreacted SO2 sorbent upstream of the mixing unit. Thereby the reaction in the mixing unit can be influenced beforehand if appropriate.
Through the apparatus in accordance with the invention, the procedure in accordance with the invention can be carried out in an efficient, inexpensive, and resource-sparing manner.